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www.ck12.org Chapter 23. MS Observing and Exploring Space

23.1 Telescopes

Lesson Objectives

• Explain how astronomers use light to study the universe beyond Earth.• Describe some different types of telescopes.• Discuss what we have learned by using telescopes.

Vocabulary

• constellation• electromagnetic radiation• electromagnetic spectrum• frequency• gamma rays• infrared light• light-year• microwaves• planet• radio telescope• radio waves• reflecting telescope• refracting telescope• space telescope• spectrometer• ultraviolet• wavelength• visible light• X rays

Introduction

Many scientists can touch the materials they study. Most can do experiments to test those materials. Biologists cancollect cells, seeds, or sea urchins to study in the laboratory. Physicists can test the strength of metal or smash atomsinto each other. Geologists can chip away at rocks and test their chemistry. But astronomers study the universe farbeyond Earth. They have to observe their subjects at a very large distance! A meteorite that lands on Earth is one ofthe few actual objects that astronomers could study.

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23.1. Telescopes www.ck12.org

Electromagnetic Spectrum

Earth is just a tiny speck in the universe. Our planet is surrounded by lots of space. Light travels across empty space.Astronomers can study light from stars to learn about the universe. Light is the visible part of the electromagneticspectrum. Astronomers use the light that comes to us to gather information about the universe.

The Speed of Light

In space, light travels at about 300,000,000 meters per second (670,000,000 miles per hour). How fast is that? Abeam of light could travel from New York to Los Angeles and back again nearly 40 times in just one second. Evenat that amazing rate, objects in space are so far away that it takes a lot of time for their light to reach us. Even lightfrom the nearest star, our sun, takes about 8 minutes to reach Earth.

Light-Years

We need a really big unit to measure distances out in space because distances between stars are so great. A light-year, 9.5 trillion kilometers (5.9 trillion miles), is the distance that light travels in one year. That’s a long way! Outin space, it’s actually a pretty short distance.

Proxima Centauri is the closest star to us after the Sun. This near neighbor is 4.22 light-years away. That meansthe light from Proxima Centauri takes 4.22 years to reach us. Our galaxy, the Milky Way Galaxy, is about 100,000light-years across. So it takes light 100,000 years to travel from one side of the galaxy to the other! It turns out thateven 100,000 light years is a short distance. The most distant galaxies we have detected are more than 13 billionlight-years away. That’s over a hundred-billion-trillion kilometers!

Looking Back in Time

When we look at stars and galaxies, we are seeing over great distances. More importantly, we are also seeing backin time. When we see a distant galaxy, we are actually seeing how the galaxy used to look. For example, theAndromeda Galaxy, shown in Figure 23.1, is about 2.5 million light-years from Earth. When you see an image ofthe galaxy what are you seeing? You are seeing the galaxy as it was 2.5 million years ago!

Since scientists can look back in time they can better understand the Universe’s history. Check out http://science.nasa.gov/headlines/y2002/08feb_gravlens.htm to see how this is true.

Electromagnetic Waves

Light is one type of electromagnetic radiation. Light is energy that travels in the form of an electromagnetic wave.Figure 23.2 shows a diagram of an electromagnetic wave. An electromagnetic (EM) wave has two parts: an electricfield and a magnetic field. The electric and magnetic fields vibrate up and down, which makes the wave.

The wavelength is the horizontal distance between two of the same points on the wave, like wave crest to wavecrest. A wave’s frequency measures the number of wavelengths that pass a given point every second. As wavelengthincreases, frequency decreases. This means that as wavelengths get shorter, more waves move past a particular spotin the same amount of time.

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FIGURE 23.1The Andromeda Galaxy as it appeared2.5 million years ago. How would you findout how it looks right now?

FIGURE 23.2An electromagnetic wave has oscillatingelectric and magnetic fields.

The Electromagnetic Spectrum

Visible light is the part of the electromagnetic spectrum (Figure 23.3) that humans can see. Visible light includes allthe colors of the rainbow. Each color is determined by its wavelength. Visible light ranges from violet wavelengthsof 400 nanometers (nm) through red at 700 nm.

There are parts of the electromagnetic spectrum that humans cannot see. This radiation exists all around you. Youjust can’t see it! Every star, including our Sun, emits radiation of many wavelengths. Astronomers can learn a lotfrom studying the details of the spectrum of radiation from a star.

Many extremely interesting objects can’t be seen with the unaided eye. Astronomers use telescopes to see objectsat wavelengths all across the electromagnetic spectrum. Some very hot stars emit light primarily at ultraviolet

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FIGURE 23.3The electromagnetic spectrum from radio waves to gamma rays.

wavelengths. There are extremely hot objects that emit X-rays and even gamma rays. Some very cool stars shinemostly in the infrared light wavelengths. Radio waves come from the faintest, most distant objects.

To learn more about stars’ spectra, visit http://www.colorado.edu/physics/PhysicsInitiative/Physics2000/quantumzone/.

Types of Telescopes

Optical Telescopes

Humans have been making and using magnifying lenses for thousands of years. The first telescope was built byGalileo in 1608. His telescope used two lenses to make distant objects appear both nearer and larger.

Telescopes that use lenses to bend light are called refracting telescopes, or refractors (Figure 23.4). The earliesttelescopes were all refractors. Many amateur astronomers still use refractors today. Refractors are good for viewingdetails within our solar system. Craters on the surface of Earth’s Moon or the rings around Saturn are two suchdetails.

Around 1670, Sir Isaac Newton built a different kind of telescope. Newton’s telescope used curved mirrors insteadof lenses to focus light. This type of telescope is called a reflecting telescope, or reflector (see Figure 23.5). Themirrors in a reflecting telescope are much lighter than the heavy glass lenses in a refractor. This is important becausea refracting telescope must be much stronger to support the heavy glass.

It’s much easier to precisely make mirrors than to precisely make glass lenses. For that reason, reflectors can bemade larger than refractors. Larger telescopes can collect more light. This means that they can study dimmer ormore distant objects. The largest optical telescopes in the world today are reflectors. Telescopes can also be madeto use both lenses and mirrors.

For more on how telescopes were developed, visit http://galileo.rice.edu/sci/instruments/telescope.html.

Radio Telescopes

Radio telescopes collect radio waves. These telescopes are even larger telescopes than reflectors. Radio telescopeslook a lot like satellite dishes. In fact, both are designed to collect and focus radio waves or microwaves from space.

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FIGURE 23.4Refracting telescopes can be very large.

FIGURE 23.5Newtonian reflector telescopes are fairly easy to make. These telescopescan be built by school students.

The largest single radio telescope in the world is at the Arecibo Observatory in Puerto Rico (see Figure 23.6).This telescope is located in a natural sinkhole. The sinkhole formed when water flowing underground dissolved thelimestone. This telescope would collapse under its own weight if it were not supported by the ground. There is abig disadvantage to this design. The telescope can only observe the part of the sky that happens to be overhead at agiven time.

A group of radio telescopes can be linked together with a computer. The telescopes observe the same object. Thecomputer then combines the data from each telescope. This makes the group function like one single telescope. Anexample is shown in Figure 23.7.

To learn more about radio telescopes and radio astronomy in general, go to http://www.nrao.edu/whatisra/index.shtml.

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FIGURE 23.6The radio telescope at the Arecibo Obser-vatory in Puerto Rico.

FIGURE 23.7The Very Large Array in New Mexico con-sists of 27 radio telescopes.

Space Telescopes

Telescopes on Earth all have one big problem: Incoming light must pass through the atmosphere. This blocks somewavelengths of radiation. Also, motion in the atmosphere distorts light. You see this when you see stars twinklingin the night sky. Many observatories are built on high mountains. There is less air above the telescope, so there isless interference from the atmosphere. Space telescopes avoid such problems completely since they orbit outsidethe atmosphere.

The Hubble Space Telescope is the best known space telescope. Hubble is shown in Figure 23.8. Hubble beganoperations in 1994. Since then it has provided huge amounts of data. The telescope has helped astronomers answermany of the biggest questions in astronomy.

The National Aeronautics and Space Administration (NASA) has placed three other major space telescopes in orbit.Each uses a different part of the electromagnetic spectrum. The James Webb Space Telescope will launch in 2014.

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FIGURE 23.8The Hubble Space Telescope has opened up the universe to humanobservation.

The telescope will replace the aging Hubble.

To learn more about NASA’s great observatories, check out http://www.nasa.gov/audience/forstudents/postsecondary/features/F_NASA_Great_Observatories_PS.html.

FIGURE 23.9Stars in the star cluster appear as points of light. Observations like thesemust be made with a space telescope.

Observations with Telescopes

Before Telescopes

Humans have been studying the night sky for thousands of years. Knowing the motions of stars helped people keeptrack of seasons. With this information they could know when to plant crops. Stars were so important that thepatterns they made in the sky were named. These patterns are called constellations. Even now, constellations helpastronomers know where they are looking in the night sky.

The ancient Greeks carefully observed the locations of stars in the sky. They noticed that some of the “stars” movedacross the background of other stars. They called these bright spots in the sky planets. The word in Greek means“wanderers.” Today we know that the planets are not stars. They are objects in the solar system that orbit the Sun.Ancient astronomers made all of their observations without the aid of a telescope.

Galileo’s Observations

In 1610, Galileo looked at the night sky through the first telescope. This tool allowed him to make the followingdiscoveries (among others):

• There are more stars in the night sky than the unaided eye can see.• The band of light called the Milky Way consists of many stars.• The Moon has craters (see Figure 23.10).• Venus has phases like the Moon.

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• Jupiter has moons orbiting around it.• There are dark spots that move across the surface of the Sun.

FIGURE 23.10Galileo made the drawing on the left in1610. On the right is a modern photo-graph of the Moon.

Galileo’s observations made people think differently about the universe. They made them think about the solarsystem and Earth’s place in it. Until that time, people believed that the Sun and planets revolved around Earth. Onehundred years before Galileo, Copernicus had said that the Earth and the other planets revolved around the Sun. Noone would believe him. But Galileo’s observations through his telescope proved that Copernicus was right.

Observations with Modern Telescopes

Galileo’s telescope got people to think about the solar system in the right way. Modern tools have also transformedour way of thinking about the universe. Imagine this: Today you can see all of the things Galileo saw using a goodpair of binoculars. You can see sunspots if you have special filters on the lenses. (Never look directly at the Sunwithout using the proper filters!) With the most basic telescope, you can see polar caps on Mars, the rings of Saturn,and bands in the atmosphere of Jupiter.

You can see many times more stars with a telescope than without a telescope. Still, stars seen in a telescope looklike single points of light. They are so far away. Only the red supergiant star Betelgeuse is large enough to appear asa disk. Except for our sun, of course.

Today, astronomers attach special instruments to telescopes. This allows them to collect a wide variety of data. Thedata is fed into computers so that it can be studied. An astronomer may take weeks to analyze all of the data collectedfrom just a single night!

Studying Starlight with Spectrometers

A spectrometer is a special tool that astronomers commonly use. Spectrometers allow them to study the light froma star or galaxy. A spectrometer produces a spectrum, like the one shown in Figure 23.11. A prism breaks light intoall its colors. Gases from the outer atmosphere of a star absorb light. This forms dark lines in the spectrum. Thesedark lines reveal what elements the star contains.

Astronomers use the spectrum to learn even more about the star. One thing they learn is how hot the star is. Theyalso learn the direction the star is going and how fast. By carefully studying light from many stars, astronomers

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FIGURE 23.11The dark lines indicate the elements that this star contains.

know how stars evolve. They have learned about the distribution and kinds of matter found throughout the universe.They even know something about how the universe might have formed.

To find out what you can expect to see when looking through a telescope, check out http://www.astronomics.com/main/category.asp/catalog_name/Astronomics/category_name/V1X41SU50GJB8NX88JQB360067/Page/1.

Lesson Summary

• Astronomers study light from stars and galaxies.• Light travels at 300,000,000 meters per second—faster than anything else in the universe.• A light-year is a unit of distance equal to the distance light travels in one year, 9.5 trillion kilometers.• When we see a star or galaxy, we see them as they were in the past, because their light has been traveling to

us for many years.• Light is energy that travels as a wave.• Visible light is part of the electromagnetic spectrum.• Telescopes make distant objects appear both nearer and larger. You can see many more stars through a

telescope than with the unaided eye.• Optical telescopes are designed to collect visible light. The two types of optical telescopes are reflecting

telescopes and refracting telescopes.• Radio telescopes collect and focus radio waves from distant objects.• Space telescopes are telescopes orbiting Earth. They can collect wavelengths of light that are normally blocked

by the atmosphere.• Galileo was the first person known to use a telescope to study the sky. His discoveries helped change the way

humans think about the universe.• Modern telescopes collect data that can be stored on a computer.• A spectrometer produces a spectrum from starlight. Astronomers can learn a lot about a star by studying its

spectrum.

Lesson Review Questions

Recall

1. Define what is one “light year.” What is a light year in numbers? Don’t forget the units!

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2. What is the speed of light? Why is this important to astronomers?

3. How do refracting telescopes work?

4. What are constellations? Why were they important to ancient people?

5. What did Galileo observe about Jupiter?

Apply Concepts

6. Picture the visible light spectrum. Where do ultraviolet wavelengths fall? Where do infrared wavelengths fall?

7. You look through a telescope at Rigel. Rigel is the brightest star in the Orion constellation. Rigel is around 800light years from Earth. What are you looking at when you look through that telescope? What does Rigel look liketoday?

8. What can you learn from studying starlight through a spectrometer?

Think Critically

9. Why do astronomers need to look at more than just visible objects when studying space? What can they learnfrom objects in other wavelengths of radiation?

10. Identify four regions of the electromagnetic spectrum that astronomers use when observing objects in space.

11. How do reflecting telescopes work? What are the advantages and disadvantages of reflecting telescopes overrefracting telescopes?

12. If you wanted to study the most distant galaxies what sort of tool would you design and why?

Points to Consider

• Radio waves are used for communicating with spacecraft. A round-trip communication from Earth to Marstakes anywhere from 6 to 42 minutes. What challenges might this present for sending unmanned spacecraftand probes to Mars?

• The Hubble Space Telescope is a very important source of data for astronomers. The fascinating and beautifulimages from the Hubble also help to maintain public support for science. However, the Hubble is growingold. Missions to service and maintain the telescope are extremely expensive and put the lives of astronauts atrisk. Do you think there should be another servicing mission to the Hubble?

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23.1 telescopes - mrsbyrdplanet.weebly.com€¦ · 23.1 telescopes lesson objectives • explain...

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